Attest: Increase satck size for level 2 isolation

Increases the stack size of initial attestation partition
to be able to execute the regression tests suite in
isolation level 2.

Change-Id: I9b4b52d3f983d7c5b5987cd8017c4ca8a6474cef
Signed-off-by: Tamas Ban <tamas.ban@arm.com>